This invention relates to electric rotating machines and more particularly to such machines in which the rotor of the machine runs in discrete increments or steps.
The present invention, while of general application, is particularly well suited for use as a stepper motor. Stepper motors are precision devices but heretofore have been susceptible to step angle performance inaccuracies due to the variations in the physical and magnetic charcteristics of the stator and the rotor. In a typical stepper motor (e.g. 1.8 mechanical degrees per step), it is advantageous to maintain the step angle accuracy to a better than .+-.3% total error per step (i.e. .+-.0.054.degree. or 0.degree.-3'-15"). The processes required to produce the parts to the required tolerances are extremely difficult to achieve. As an example, a 1.8.degree. motor that requires fifty stator teeth with an internal diameter of 0.860" has a tooth width requirement of 0.020" and a true angular position requirement of 7.2.degree.. To achieve the step angle accuracy, the parts must be produced to tolerances of 0.020.+-.0.00006" for the tooth width and angle tolerances of 7.2.degree..+-.0.degree.-12'-58". The air gap between the rotor and the stator must also have very accurate diametrical and circular dimensions.
Several types of prior stepper motors have two or more stator windings which are energized in different phase relationship with each other. Without perfect symmetry between the phases of the windings, the rotor will step with different step angles, i.e. "long-step", "short-step", and these angles will vary with the direction of rotation of the rotor.
Another problem with stepper motors of the type previously employed is excessive eddy current losses. The net effect of eddy current losses in a stepper motor is the creation of different inductance components which have a deleterious effect on the step angle accuracy and torque depending upon which phase is energized.
Many prior stepper motors had an additional problem in that the top and bottom housings of the motor were difficult to align by means of mounting screws needed to attach the two housings together. Additionally, it was important not to locate the mounting screws in a position which would create a magnetic imbalance.
The present invention is directed toward solving these problems and provides a workable and economical solution to them.